Typical wireless communications systems, including cellular telephones, radios, and other wireless systems communicate data at high frequencies, i.e, at radio frequency (RF). Radio frequency signals are electrical signals conveying useful information having a frequency from about 3 kilohertz (kHz) to thousands of gigahertz (GHz), regardless of the medium through which such signals are conveyed. Thus an RF signal may be transmitted through air, free space, coaxial cable, fiber optic cable, etc. To process RF signals receive circuitry of a receiver, for example, generally converts the received RF signals to one or more lower frequencies, including an intermediate frequency (IF) and a baseband frequency. As an example, in a radio tuner, a frequency corresponding to a desired radio channel is tuned by mixing an incoming RF signal spectrum with a frequency generated in a local oscillator (LO) to obtain signal information of the desired channel. In various implementations, such a LO may be a voltage controlled oscillator or an NCO, such as a digitally controlled oscillator (DCO).
A VCO is typically included in a phase-locked loop (PLL) circuit to generate the desired LO signal based upon a feedback loop determined with reference to phase information. However, such systems often suffer from phase noise and other problems. Accordingly, some systems instead implement an NCO, which may be controlled using frequency information.
In practice, a controlled oscillator can have its frequency controlled by changing capacitance values of one or more capacitors coupled to an oscillator element, such as a resonant tank. By varying the capacitance, the frequency generated by the controlled oscillator may be correspondingly varied. To effect frequency tuning, one or more capacitor banks may be provided. Each capacitor bank may include one or more capacitors to be switched into or out of a capacitance array line to affect the total capacitance. By controlling the capacitance, the frequency of the controlled oscillator may be concomitantly controlled.
Analog control of capacitances is often effected using an analog varactor to continuously adjust capacitance values. Other implementations use a digital word to control a capacitor bank that includes an array of capacitors to be switched into or out of a capacitor array line. In practice, the range of capacitances needed to cover a given frequency range, as well as provide small enough frequency steps for proper tuning, can be difficult to design and fabricate.
Capacitor array banks are typically formed of a plurality of capacitor branches coupled in parallel between an input node (i.e., a capacitor array line) and a ground potential. In embodiments that are discretely controlled, a digital control word may include a plurality of bits, ranging from a most significant bit (MSB) to a least significant bit (LSB), each to control a respective branch of the array bank, each branch of which may have a different capacitance value. To maintain operation at a high frequency, arbitrary fixed capacitors cannot be added into a capacitor bank. The MSBs dominate the loss of the system. To have predictable changes, especially in the LSBs, there must be a similar structure across the capacitor bank. However, a similar structure for all capacitors is not easily controlled, as significant variances can exist between the large and small capacitor values. These significant variances can negatively impact performance by leading to frequency gaps within a desired range.
Accordingly, a need exists to provide for improved control of oscillators, and particularly to control of fine frequency changes in a controlled oscillator.